Need help: Is the Condom his of his sisters

So last night I was getting me and my boyfriends clothes out of the dryer and at the bottom on the dryer I noticed a condom. His sisters lives with us and washes her clothes in the dryer as well and before hand her clothes were in the dryer before our pile of clothes had dried. He said the condom has to be hers and I asked her and she said they are hers and that she has a bunch of them... Not sure if they are really hers or if she’s taking up for him… Ik his sister is very active with a lot of guys so it’s a possibility it could be hers. I’ve never caught my Fiancé with condoms before so it’s a possibility it may be a coincidence. But I don’t want to make the wrong decision by leaving if it’s really hers. We were about to get married in 2 weeks so not sure if this is coincidence or I’m catching him in a lie….